Embodiments of the present invention relate generally to gas analysis and more particularly to the measurement of gas density to determine gas flux.
When gas density measurements are performed by scanning a single rotational line or a few discrete lines of analyte with a single-mode tunable laser source, the measured signal is temperature and pressure dependent due to the combined effects of Boltzmann population distribution of rotational levels and Doppler broadening and temperature-dependent pressure broadening of individual lines. These pressure and temperature effects can also be affected by the presence of water and other gases in the sampled air. The combination of all of these effects are referred to herein as T-P (temperature-pressure) effects. In addition, if a constant mixing ratio gas is used at a relatively constant pressure, then the measured gas density itself changes with temperature and water content due to thermal expansion and water dilution of the gas per the Ideal Gas Law.
When the temperature or water content of the gas changes, the T-P effects may lead to a large change in absorption, significantly affecting the gas density measurement. In general, the T-P effects are specific and different for each absorption line of each gas.
For slow measurements of gas density (e.g., measurements taken on the order of seconds and longer per measurement), the T-P effects can be easily calibrated out because the mean temperature and water content of the gas in the sampling volume can be easily measured. For the case of fast measurements of gas density (.e.g., several measurements taken per second), it is difficult to correct for T-P effects on-the-fly, because it would require accurate and precise measurements of gas temperature and water content integrated over the entire sampling volume. Moreover, the gas temperature and water content measurements would have to be recorded at the same moment when laser absorption due to gas density is measured.
Existing gas analyzers, especially for trace gases such as methane, nitrous oxide, isotopes of carbon dioxide and water, etc., are closed-path sensors requiring long intake tubes and powerful pumps to allow sample gas flow of 30-100 lpm (liters per minute) and more. The fast temperature changes are attenuated in these long intake tubes so slow temperature measurements can be used, but power consumption of such sensors systems goes up to 1000 Watts and more, making them difficult to use in remote locations where most of the natural gas exchange processes for these gases occurs.